Differential conductivity recirculation monitor

ABSTRACT

A differential conductivity recirculation monitor quantitatively determines the degree of recirculation in a fistula by comparing the conductivity of blood entering the fistula to the conductivity of blood being withdrawn from the fistula. A discrete quantity of a high conductivity marker fluid is injected into the blood entering the fistula, altering the conductivity of the blood entering the fistula. The altered conductivity blood enters the fistula and, if recirculation is present, co-mingles with blood in the fistula, altering the conductivity of the blood ion the fistula in proportion to the degree of recirculation. Blood withdrawn from the fistula has an altered conductivity related to the degree of recirculation. Quantitative values of the conductivity of the altered conductivity blood entering the fistula and the conductivity of the blood being withdrawn from the fistula are measured and a difference determined. The determined difference between the conductivities is used to determine a quantitative measurement of the degree of recirculation in the fistula.

This application is a divisional application of U.S. patent applicationSer. No. 08/332,647, filed Nov. 1, 1994, which is a continuation of U.S.Pat. application Ser. No. 07/954,584, filed Sep. 30, 1992, nowabandoned.

FIELD OF THE INVENTION

This invention relates to measurement of recirculation efficiency. Moreparticularly, this invention relates to measurement of the recirculationefficiency of a biological or medical fluid during a medical procedureor for diagnostic purposes.

BACKGROUND OF THE INVENTION

In many medical situations it is desirable to quantitatively determine,or measure, the recirculation rate or the recirculation efficiency of abiological or medical fluid to increase the benefits of, or decrease thetime required for, a therapeutic treatment, or for diagnostic purposes.For example, hemodialysis (herein "dialysis") is an inconvenient,expensive, and uncomfortable medical procedure. It is, therefore, widelyrecognized as desirable to minimize the amount of time required tocomplete the procedure and to achieve a desired level of treatment.

In dialysis, a joint is typically surgically created between a vein andan artery of a patient undergoing dialysis. The joint provides a bloodaccess site where an inlet line toga dialysis apparatus and an outletline from the dialysis apparatus are connected. The inlet line drawsblood to be treated from the patient, while the outlet line returnstreated blood to the patient.

This joint may be an arteriovenous fistula, which is a direct connectionof one of the patient's veins to one of the patient's arteries.Alternatively the joint may be a synthetic or animal organ graftconnecting the vein to the artery. As used herein, the term "fistula"refers to any surgically created or implanted joint between one of thepatient's veins and one of the patient's arteries, however created.

In the fistula a portion of the treated blood returned to the patient bythe outlet line may recirculate. Recirculating treated blood willco-mingle with untreated blood being withdrawn from the patient by theinlet line. This recirculation, and the resulting co-mingling of treatedand untreated blood, is dependent, in part, on the rate at which bloodis withdrawn from and returned to the patient. The relationship istypically a direct, but non-linear relationship. It can be readilyappreciated that the dialysis apparatus will operate most effectively,and the desired level of treatment achieved in the shortest period oftime, when the inlet line is drawing only untreated blood at the maximumflow rate capability of the dialysis apparatus consistent with patientsafety. As a practical matter, however, as flow rate through thedialysis apparatus is increased, the proportion of recirculated treatedblood in the blood being drawn through the inlet line is increased. Inorder to select the flow rate through the dialysis apparatus, it isdesirable to know the proportion of recirculated treated blood in theblood being withdrawn from the patient by the inlet line. Thisproportion is referred to herein as the "recirculation ratio". Therecirculation ratio can also be defined as the ratio between the flow ofrecirculated blood being withdrawn from the fistula to the flow of bloodbeing returned to the fistula. Recirculation efficiency may then bedefined by the relationship:

    E=1-R

where

E=Recirculation efficiency

R=Recirculation ratio

Alternatively, recirculation efficiency may be equivalently expressed asthe ratio of blood flow being returned to the fistula, but not beingrecirculated, to the total blood flow being returned to the fistula.Knowing the recirculation efficiency, the dialysis apparatus operatorcan adjust the flow rate through the dialysis apparatus to minimize thetime required to achieve the desired level of treatment.

In the prior art, quantitative determination of recirculation ratio orrecirculation efficiency has typically required laboratory testing, suchas blood urea nitrogen tests, which take considerable amounts of timeand which require withdrawing blood from the patient, which isrecognized as undesirable.

A method and apparatus for qualitatively detecting the presence orabsence of recirculation in a fistula is described in "FAM 10 FistulaFlow Studies and their Interpretation" published by Gambro, Ltd. basedon research performed in 1982. The Gambro method and apparatus injects aquantity of a fluid having an optical density less than the opticaldensity of treated blood into the dialysis apparatus outlet line. Aresulting change in the optical density of the blood being drawn throughthe dialysis apparatus inlet line is qualitatively detected asindicative of the presence of recirculation. The Gambro method andapparatus does not quantitatively determine or measure a recirculationratio or recirculation efficiency.

Devices which qualitatively determine recirculation by thermaltechniques are also known.

A quantitative measurement of the recirculation efficiency of a bodilyor medical fluid is useful in other therapeutic and diagnosticprocedures as well. For example, recirculation ratios and efficienciesare useful for determining cardiac output, intervascular recirculation,recirculation in non-surgically created access sites, and dialyzerperformance from either the blood side or the dialysate side of thedialyzer, or both.

It is known that the electrical conductivity of a fluid in a closednon-metallic conduit can be measured without contact with the fluid byinducing an alternating electrical current in a conduit loop comprisinga closed electrical path of known cross sectional area and length. Themagnitude of the current thus induced is proportional to theconductivity of the fluid. The induced current magnitude may then bedetected by inductive sensing to give a quantitative indication of fluidconductivity. A conductivity cell for measuring the conductivity of afluid in a closed conduit without contact with the fluid is described inU.S. Pat. No. 4,740,755 entitled "Remote Conductivity Sensor HavingTransformer Coupling In A Fluid Flow Path," issued Apr. 26, 1988 toOgawa and assigned to the assignee of the present invention, thedisclosure of which is hereby incorporated herein by reference.

It is against this background that the differential conductivityrecirculation monitor of the present invention developed.

SUMMARY OF THE INVENTION

A significant aspect of the present-invention is a method and apparatusfor quantitatively measuring the recirculation efficiency of a fluid ina vessel, such as a fistula, containing the fluid and from which fluidis being withdrawn through a first, or inlet, line and to which thefluid is being returned through a second, or outlet, line. In accordancewith this aspect of the invention a discrete quantity (referred toherein as a "bolus") of a marker fluid having an electrical conductivitydifferent from the electrical conductivity of the fluid flowing in theoutlet line is injected into the fluid flowing in the outlet line. Themarker fluid alters the conductivity of the fluid in the outlet line.The altered conductivity fluid in the outlet line enters the vessel and,if recirculation is present, co-mingles with the fluid in the vessel.The conductivity of the fluid in the vessel is thus altered inproportion to the degree of recirculation. The inlet line withdraws fromthe vessel fluid having an altered conductivity related to the degree ofrecirculation. Quantitative values of the conductivity of the alteredconductivity fluid in the outlet line and the conductivity of the fluidin the inlet line are measured and a difference determined. Thedetermined difference between the conductivity is used to determine aquantitative measurement of the recirculation efficiency in the vessel.

Another significant aspect of the present invention is a method andapparatus for quantitatively determining a difference in theconductivity of a first and a second fluid. In accordance with thisaspect of the invention the first fluid is placed in a firstconductivity cell and the second fluid is placed in a secondconductivity cell. Each conductivity cell is a conduit of predeterminedcross sectional area formed into a ring-like configuration ofpredetermined circumference. The configuration of each conductivity cellconfines the fluid therein into an electrical path having apredetermined cross sectional area and a predetermined path length. Anexcitation coil excited by an alternating electrical current isproximate to the conduits of both conductivity cells at an excitationlocation. A first fluid alternating current is induced in the firstfluid in relation to the conductivity of the first fluid. A second fluidalternating current is induced in the second fluid in relation to theconductivity of the second fluid. The first and second fluid alternatingcurrents flow in the first and second fluids in Substantially the sameelectrical direction at the excitation location. A sensing coil isproximate to the conduits of the first and second conductivity cells ata sensing location. The tubes of the first and second conductivity cellsare oriented at the sensing location so that the first fluid alternatingcurrent flows in a substantially opposite electrical direction from thedirection of the second fluid alternating current. A sensed alternatingcurrent is induced in the sensing coil by the first fluid alternatingcurrent and the second fluid alternating current that is proportional tothe difference between the first and second fluid alternating currents.The sensed alternating current is quantitatively indicative of thedifference between the conductivities of the first and second fluids.

Still another significant aspect of the present invention is a tubingset adapted for use with an apparatus incorporating the presentinvention. In accordance with this aspect of the invention a disposabletubing set, such as are typically used in dialysis apparatuses and othermedical apparatuses, has a conductivity cell formed into each of twotubes of the tubing set. Each conductivity cell is a conduit ofpredetermined cross sectional area formed into a ring-like ofpredetermined circumference. Each conductivity cell has formed to it anupstream tube in fluid communication with the conductivity cell and adownstream tube in fluid communication with the conductivity cell, theupstream and downstream tubes being oriented so that the ring-likeconfiguration of the conductivity cell forms two tubing branches fluidlyinterconnecting the upstream tube to the downstream tube.

A further significant aspect of the present invention is a medicalapparatus incorporating the recirculation monitor.

A more complete appreciation of the present invention and its scope canbe obtained from understanding the accompanying drawings, which arebriefly summarized below, the following detailed description of apresently preferred embodiment of the invention, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a dialysis system incorporating adifferential conductivity recirculation monitor in accordance with thepresent invention.

FIG. 2 is a partial perspective view illustrating the functionalelements of the differential conductivity recirculation monitor shown inFIG. 1.

FIG. 3 is an electrical schematic diagram of the differentialconductivity recirculation monitor shown in FIG. 2.

FIG. 4 is an electrical block diagram of sensing logic usable with thedifferential conductivity recirculation monitor illustrated in FIGS. 2and 3.

FIG. 5 is a graph illustrating differential conductivity versus timeduring a recirculation test employing the differential conductivityrecirculation monitor shown in FIG. 2.

FIG. 6 is a graph illustrating the integral of differential conductivityversus time during a recirculation test employing the differentialconductivity recirculation monitor shown in FIG. 2, having substantiallythe same time scale as FIG. 5.

FIG. 7 is a partial elevational view of a tubing set and sectional viewof an excitation and sensing unit for use with the dialysis system shownin FIG. 1, incorporating the differential conductivity recirculationmonitor in accordance with the present invention.

FIG. 8 is a partially diagrammatic sectional view taken substantially atline 8--8 in FIG. 7.

FIG. 9 is a partially diagrammatic perspective view of the excitationand sensing unit of the differential conductivity recirculation monitorof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a dialysis system 20 incorporating a differentialconductivity recirculation monitor 22 for determining and displayingrecirculation efficiency in accordance with the present invention. Thedialysis system 20, which is one example of a medical system with whichthe present invention may be advantageously used, comprises a dialysisapparatus 24 connected to a fistula 26 surgically formed in a dialysispatient (not shown). Untreated blood is drawn from the fistula 26through a dialyzer inlet needle 28 and a dialyzer inlet line 30. Treatedblood is returned to the fistula through a dialyzer outlet line 32 and adialyzer outlet needle 34. The recirculation monitor 22 is located inthe dialyzer inlet and outlet lines 30 and 32 at a point intermediatebetween the fistula 26 and the dialysis apparatus 24.

The dialysis apparatus 24 comprises a blood pump 36 typically aperistaltic pump, a dialyzer 38 having a blood compartment 40 and adialysate compartment 42 separated by a semi-permeable membrane 44, abubble trap 46 and a dialysate generator 48. Blood is drawn from thefistula 26 by the action of the blood pump 36 and passed through theblood compartment 40 of the dialyzer 38. The membrane 44 allows transferof impurities in the blood, such as urea and creatinine, from the bloodcompartment 40 to the dialysate compartment 42 of the dialyzer 38. Thedialysate compartment 42 is connected to a dialysate generator 48 whichgenerates the dialysate, a liquid isotonic to blood, and circulates itthrough the dialysate compartment 44.

The principles of operation of the differential conductivityrecirculation detector 22 of the present invention are explained inconjunction with FIGS. 2 and 3. The recirculation detector 22 comprisesa needle access site 50 in the dialyzer outlet line 32. A first oroutlet conductivity cell 52 is located in the dialyzer outlet line 32downstream of the needle access site 50. A second or inlet conductivitycell 54 is located in the dialyzer inlet line 30. The first conductivitycell 52 comprises an upstream connection 56, a downstream connection 58and first and second tubing branches 60 and 62, respectively, each ofwhich interconnect the upstream connection 56 with the downstreamconnection 58. Treated blood from the dialyzer flows in the dialyzeroutlet line 32 through the needle access site 50 to the upstreamconnection 56. At the upstream connection 56 the flow splitsapproximately equally with a portion of the treated blood flowing ineach of the two tubing branches 60 and 62 of the outlet conductivitycell 52. The flow rejoins at the downstream connection 58 and flowsthrough the dialyzer outlet line 32 to the fistula 26 (FIG. 1).Similarly, the inlet conductivity cell 54 comprises an upstreamconnection 64, a downstream connection 66 and third and fourth tubingbranches 68 and 70, respectively, which each connect the upstreamconnection 64 to the downstream connection 66. Untreated blood from thefistula 26 flowing in the dialyzer inlet line 30, enters the inletconductivity cell 54 at the upstream connection 64 divides approximatelyequally between the third and fourth tubing branches 68 and 70 andrejoins at the downstream connection 66 to the inlet conductivity cell54. Each one of the tubing branches 60, 62, 68 and 70 has the same crosssectional area and length as each other one of the tubing branches.

The blood, or other biological or medical fluid, flowing in eachconductivity cell 52 and 54 comprises an electrical circuit. Theelectrical circuit is a path for circulation of an electrical currentfrom the upstream connection, through one of the tubing branches, to thedownstream connection and from the downstream connection through theother one of the tubing branches to the upstream connection.

The outlet conductivity cell 52 and the inlet conductivity cell 54 arepositioned adjacent to each other in an angular relationship resemblinga pretzel so that the first tubing branch 60 of the outlet conductivitycell 52 is positioned parallel to the third tubing branch 68 of theinlet conductivity cell at an excitation location. The conductivitycells are further positioned so that the second tubing branch 62 of theoutlet conductivity cell 52 crosses the fourth tubing branch 70 of theinlet conductivity cell 54 at an angle, approximately sixty degrees inthe preferred embodiment, at a sensing location. An excitation coil 72encircles the first tubing branch 60 of the outlet conductivity cell 52and the third tubing branch 68 of the inlet conductivity cell 54 at theexcitation location. A sensing coil 74 encircles the second tubingbranch 62 of the outlet conductivity cell 52 and the fourth tubingbranch 70 of the inlet conductivity cell 54 at the sensing location.

An electrical circuit, as is illustrated schematically in FIG. 3, isthus formed. The excitation coil 72 is inductively coupled to the outletconductivity cell 52 and the inlet conductivity cell 54. When a sourceof excitation energy 76 causes an alternating excitation current,illustrated by direction arrow 78, to flow in the excitation coil 72 achanging magnetic field is generated which causes an electrical current,illustrated by the direction arrow 80, to flow in the blood in theoutlet conductivity cell 52 and causes another electrical current,illustrated by direction arrow 82, to flow in the same electricaldirection in the blood in the inlet conductivity cell 54. Since theconductivity cells 52 and 54 are formed to create electrical paths ofequal cross sectional area and equal path length the electricalconductance of the paths, as illustrated by the schematic resistors 84and 86, and thus the magnitude of the induced currents 80 and 82, willbe related to the conductivity of the blood in the respectiveconductivity cells 52 and 54.

The induced currents 80 and 82 flowing in the outlet and inletconductivity cells 52 and 54 generate a changing magnetic field at thesensing location that induces a sensed current, illustrated by directionarrow 88, in the sensing coil 74. The induced currents 80 and 82 are inopposite electrical directions so that the magnetic field at the sensinglocation has a magnitude proportional to the difference between theinduced currents. The sensed current 88 is proportional to the magneticfield at the sensing location where the sensing coil 74 encircles thesecond and fourth tubing branches 62 and 70, respectively. The sensedcurrent 88 induced in the sensing transformer 74 is thereforeproportional to a difference between the induced currents 80 and 82 inthe outlet and inlet conductivity cells 52 and 54, respectively. Theinduced currents 80 and 82 in the outlet and inlet conductivity cells 52and 54, respectively, are related to the conductivity of the fluids inthose chambers. Therefore, the magnitude of the sensed current 88induced in the sensing coil 74 will be related to the difference betweenthe conductivities of the fluids in the outlet and inlet conductivitycells 52 and 54. The sensed current 88 is delivered to, and interpretedby a sensing logic and display circuit 90, which displays therecirculation efficiency.

It should be appreciated that the present invention will function insubstantially the same way if the locations of the exciting coil 72 andsensing coil 74 are reversed.

Referring now to FIGS. 1 and 2, to use the recirculation monitor 22 toperform a recirculation test the dialysis system operator injects abolus of a marker fluid into the treated blood in the dialyzer outletline 32 at the needle access site 50 using a typical hypodermic needle92. The marker fluid may have an electrical conductivity that is higheror lower than the fluid flowing in the outlet line 32. In the preferredembodiment a high conductivity marker fluid is used to avoid damagingblood cells. In the preferred embodiment the bolus is 1 milliliter of 24percent hypertonic saline solution. The conductivity of the treatedblood being returned to the patient through the dialyzer outlet line 32and the outlet conductivity cell 52 of the recirculation monitor 22 isaltered. This altered conductivity blood enters the fistula through theoutlet needle 34.

If the flow balance in the fistula 26 is such that no flow isrecirculating the altered conductivity blood will exit the fistula, asillustrated by the flow circulation arrow 94, without altering theconductivity of the blood within the fistula. If, however, the flowbalance within the fistula 26 is such that blood is recirculating, asillustrated by flow circulation arrow 96, a portion of the bloodwithdrawn from the fistula 26 by the pump 36 will be the alteredconductivity blood. The recirculation monitor 22 measures theconductivity of the blood flowing in the outlet line 32 and theconductivity of the blood flowing in the inlet line 30 andquantitatively determines the difference between those conductivitiescontinuously throughout the recirculation test. The sensing logic anddisplay circuit 90 interprets the quantitative conductivity differencesmeasured by the recirculation monitor 22 to determine recirculationefficiency.

The determination of recirculation efficiency will be explained byreference to FIGS. 4, 5 and 6. The outlet conductivity cell 52 and theinlet conductivity cell 54 may be thought of as signal generatorsgenerating the induced currents 80 and 82 in the outlet and inletconductivity cells. The induced current 82 of the inlet conductivitycell 54 is inverted 98 and added 100 to the induced current 80 in theoutlet, conductivity cell 52, by virtue of the physical relationshipsbetween the conductivity cells, excitation coil 72 and sensing coil 74,to produce the sensed current 88.

The sensing logic and display circuit 90 performs a zeroing operation102, a dialyzer outlet flow determining operation 104, andunrecirculated flow determining operation 106, and a dividing operation108, and includes a visual display device 110, preferably a liquidcrystal display. Alternatively the functions of the sensing logic anddisplay circuit 90 may be performed by a digital computer (not shown).

FIG. 5 is a graph illustrating differential conductivity (reference 112)as a function of time (reference 114) during a typical recirculationtest. FIG. 6 is a graph illustrating the integral of differentialconductivity (reference 116) as a function of time 114 during thetypical recirculation test. Prior to the beginning of the recirculationtest there may be some normal difference (reference 118) between theconductivity of the treated blood in the dialyzer outlet line 32 (FIG.2) and the untreated blood in the dialyzer inlet line 30 (FIG. 2). Thisnormal conductivity difference 118 is subtracted from the sensed current88 by the zeroing operation 102 of the sensing logic and display circuit90 to remove the effect of the normal difference in conductivity 118from determination of recirculation efficiency. The recirculation testbegins (reference time T1) when the bolus of high conductivity fluid isinjected into the dialyzer outlet line 32 (FIG. 2) at the needle accesssite 50 (FIG. 2). The conductivity of the treated blood in the dialyzeroutlet line 32 (FIG. 2) is increased. As the bolus passes through theoutlet conductivity cell 52 (FIG. 2) the differential conductivity 112increases (reference 120) and then decreases (reference 122) until thenormal conductivity difference 118 is reached (reference time T2). Theoutlet flow determining operation 104 calculates the integral ofconductivity from the start of the test (reference time T1) until thedifferential conductivity returns to the normal value 118 (referencetime T2). The integral 116 of the conductivity increases (reference 124)until a first steady state value (reference 126) of the integral 116 isreached when the differential conductivity 112 returns to the normalvalue 118 (reference time T2). The first steady state value 126 isstored by the outlet flow determining operation 104 and isrepresentative of the flow of treated blood in the dialyzer outlet line32 (FIG. 2).

After the treated blood with the altered conductivity enters the fistula26 (FIG. 1) a portion of it may recirculate and be withdrawn from thefistula 26 (FIG. 1) through the dialyzer inlet line 30 (FIG. 2). Theconductivity of the untreated blood in the inlet conductivity cell 54 isincreased (reference time T3), causing the differential conductivity todecrease 128 and then increase 130, returning to the normal value ofconductivity difference 118 (reference time T4). The integral ofdifferential conductivity from the beginning of the recirculation test(reference time T1) until the normal value of conductivity difference118 is reached again (reference time T4) is calculated by theunrecirculated flow determining operation 106 of the sensing logic anddisplay circuit 90. The integral of differential conductivity 116decreases (reference) to a second steady state value 134 (reference timeT4.

The second steady state value 134 of the integral of differentialconductivity is stored by the unrecirculated flow determining operation106 of the sensing logic and display circuit 90 and is representative ofthe portion of the bolus of high conductivity liquid that was notrecirculated. The second steady state value 134 is thus representativeof the unrecirculated portion of the treated blood flow. The dividingoperation divides the second steady state value 134 by the first steadystate value 126 to calculate a recirculation efficiency 136. Therecirculation efficiency 136 is provided to the operator as a visualoutput by the display device 110.

It will be apparent to those skilled in the art that the sensing logicand display circuit 90 may be implemented using analog or digitalcircuit devices and that other calculation algorithms may be used tocalculate recirculation efficiency 138. Further, the recirculationefficiency 138 may be calculated in real time or, alternatively, thenecessary data stored and the calculations performed on the stored data.

Further details of the preferred embodiment of the differentialconductivity recirculation monitor will be explained by reference toFIGS. 7-11.

FIG. 7 illustrates a portion of a typical disposable tubing set 140incorporating conductivity cells 52 and 54 in accordance with thepresent invention. As is well known in the art, it is highly desirablefor all portions of the tubing set 140 to be used with a dialysis systemto be disposable, in order to prevent cross contamination and infectionbetween patients. This is true of most blood and other biological ormedical fluid processing systems.

Disposable tubing sets may be formed from a plurality of plastic tubes,connectors, needles and medical devices using techniques that are wellknown in the art. The discussion of the tubing set 140 will therefore belimited to a discussion of the differential conductivity recirculationmonitor 22 (FIG. 1) portion of the tubing set.

The dialyzer outlet line 32 is a plastic tube which extends through theneedle access site 50, into the outlet conductivity cell 52. The outletconductivity cell 52 comprises a plastic conduit loop and includes theupstream connection 56, elongated divided first and second tubingbranches 60 and 62, and the downstream connector 58. The downstreamconnector 58 has mounted in it an extension of the dialyzer outlet line32, which is mounted through a connector 142 to the outlet needle 34.

The dialyzer inlet needle 28 is connected through a connector 144, tothe dialyzer inlet line 30. The dialyzer inlet line 30 is connected tothe inlet conductivity cell 54, which includes the upstream connection64, elongated divided third and fourth tubing branches 68 and 70respectively, and downstream connector 66. The dialyzer inlet line 30extends from the downstream connector 66 to the dialyzer apparatus 24(FIG. 1).

In the preferred embodiment the portion of the dialyzer outlet line 32between the dialyzer outlet needle 34 and the downstream connector 58 ofthe outlet conductivity cell 52 and the portion of the dialyzer inletline 30 between the dialyzer inlet needle 28 and the upstream connector64 of the inlet conductivity cell 54 must be sufficiently long so thatthe bolus of marker fluid passes completely through the outletconductivity cell before any altered conductivity fluid from the fistula26 enters the inlet conductivity cell.

The conductivity cells 52 and 54 have the overall shape of links in anordinary chain, straight side portions 146 being joined at their ends bysemicircular portions 148. In cross-section at the excitation location,as shown in FIG. 8, the wall of each conductivity cell 42 and 54 definesa D, the insides of the Ds providing conduit portions 150 and 152. Aflat portion 154 of the D of the outlet conductivity cell 52 is abuttedand adhered to a flat portion 156 of the D of the inlet conductivitycell 54 along one pair of semicircular portions 148 of the conductivitycells. The other pair of circular portions 148 are separated so thataxes of the conductivity cells 52 and 54 define therebetween an angle ofapproximately sixty degrees. The flat portions 154 and 156 of theconductivity cells 52 and 54 are further joined along two of thestraight portions 146 at a location along the second and fourth tubingbranches 62 and 70, respectively at the sensing location. An orientationtab 159 is formed on the inlet conductivity cell 54.

Mating with tube set 140 is a tubing set acceptor 160. As shown in FIG.9, tee tubing set acceptor 160 comprises a portion of an excitation andsensing unit 162 which also includes a logic circuit module 164. Thetubing set acceptor 160 comprises a portion of a first, or rear,acceptor plate 166 and a second, or front, acceptor plate 168 joined bya hinge 169 for motion between open and closed positions and providedwith a latch or spring (not shown) to hold the acceptor plates in theclosed position. The first acceptor 166 plate is relieved to accept intoappropriately-shaped indentations 170 thereof the outlet conductivitycell 52 (FIG. 2) and portions the tubing set 140 (FIG. 7). The secondacceptor plate 168 is relieved to accept into appropriately-shapedindentations 172 thereof the inlet conductivity cell 54 and portions ofthe tubing set 140 (FIG. 7). An orientation tab recess 173 is defined byat least one of the appropriately shaped indentations 170 and 172. Theorientation tab recess 173 cooperates with the orientation tab 159 (FIG.7) of the tubing set 140 (FIG. 7) to assure that the tubing set iscorrectly oriented when installed in the tubing set acceptor 160.

The tubing set acceptor 160 is sufficiently large to support theconductivity cells 52 and 54 and enough of the dialyzer outlet line 32and dialyzer inlet line 30 so that fluid flow patterns through theconductivity cells are substantially repeatable, being relativelyunaffected by bends, curves, tubing movement, and other disturbances orvariations in the positions of the outlet and inlet lines with respectto the conductivity cells during measurement.

The excitation coil 72 and sensing coil 74 are mounted to the tubing setacceptor 160. The excitation coil 72 and sensing coil, 74 are positionedat right angles to each other to minimize magnetic interference betweenthe coils. The excitation coil 72 comprises a first, or rear, and asecond, or front, half core 174 and 176, respectively. Similarly thesensing coil comprises a third, or rear, and a fourth, or front,half-core 178 and 180 respectively. The first and third half-cores 174and 178, respectively are mounted to the first acceptor plate 166 andthe second and third half cores 176 and 180 respectively are mounted tothe second acceptor plate 186.

As illustrated in FIG. 8, each half core has a U-shaped configuration,with short legs 182 having ends 184 and connecting legs 186. The tubingset acceptor 160 holds a portion of the tubing set 140 which includesthe conductivity cells 52 and 54 in a fixed relationship with theexcitation coil 72 and sensing coil 74.

The first and second half cores 174 and 176 are oriented so that theirends 184 abut when the first and second acceptor plates 166 and 168 arebrought to the closed position. The excitation coil 72 thus formed is inthe shape of a rectangle defining a rectangular window. The third andfourth half cores 178 and 180 are similarly oriented so that their endsabut when the first and second acceptor plates 166 and 168 are broughtto the closed position. The sensing coil 74 thus formed is also in theshape of a rectangular ring defining a rectangular window (not shown).When a tubing set 140 is placed in the tubing set acceptor 160 the firstand third tubing branches 60 and 68 are engaged in the window of theexcitation coil 72 and the second and fourth tubing branches 62 and 70are engaged in the window of the sensing coil 74 so that the coilsencircle the corresponding tubing branches. Biasing springs 188 may beprovided to hold corresponding half-cores in firm contact when theacceptor plates 166 and 168 are closed.

The legs 182 and 186 of the coil 72 and 74 are square in cross-section.At least one connecting leg 186 of each coil 72 and 74 is transformerwire wrapped 190.

The logic circuit module 164 of the excitation and sensing unit 162 maybe mounted to one of the acceptor plates 168 or may be separate from thetubing set acceptor 160 with wiring interconnections (not shown) to thetubing set acceptor 160. The logic circuit module houses the sensinglogic and display circuit 90, with the display device 110 and one ormore manual input switches 192 to enable the operator to perform suchfunctions as turning the recirculation monitor on and off, testing theoperation of the monitor and initiating recirculation test.

Although the display device 110 and manual input switches 192 are shownin FIG. 9 as being on a side 194 of the logic circuit module 164adjacent to the second acceptor plate 168, in the preferred embodimentthe display device and manual input switches may be on a side 196opposite the second acceptor plate 168, or any other side of the logiccircuit module.

The circuitry for conductivity measurement and calibration may suitablybetas set forth in the Ogawa patent incorporated by reference above.

The preferred embodiment of the present invention has been described byreference to determination of recirculation efficiency in a surgicallycreated blood access site during, or in conjunction with, a hemodialysisprocedure. It should be understood that the present invention is not solimited. The present invention may be used in a variety of medical andnon-medical circumstances where it is desirable to determinerecirculation efficiency. Further, it should be understood that thepresent invention may be used in a variety of medical and non-medicalcircumstances where it is desirable to compare the electricalconductivities of two fluids. Presently preferred embodiments of thepresent invention and many of its aspects, features and advantages havebeen described with a degree of particularity. It should be understoodthat this description has been made by way of preferred example, andthat the invention is defined by the scope of the following claims.

What is claimed is:
 1. A method for determining a difference inelectrical conductivity of at least two fluids comprising:placing afirst fluid into a first conductivity cell having a tubular ringconfiguration, said fluid forming a continuous electrical path; placinga second fluid into a second conductivity cell having a tubular ringconfiguration, said fluid forming a continuous electrical path; inducinga first electrical current in the first fluid in the first conductivitycell and a second electrical current in the second fluid in the secondconductivity cell; sensing the first electrical current in the firstfluid in the first conductivity cell and the second electrical currentin the second fluid in the second conductivity cell; and subtracting thesecond electrical current from the first electrical current to produce asignal representative of the difference in the conductivity between thefirst and the second fluids.
 2. A method as defined in claim 1 whereinthe inducing step further comprises:positioning an excitingelectromagnetic coil in proximity with the first and second conductivitycells at an exciting location, the first conductivity cell beingoriented with respect to the exciting coil at the exciting location in aparallel relationship with the second conductivity cell; inducing thefirst electrical current in an electrical direction with respect to theexciting electromagnetic coil around the path of the first conductivitycell; and simultaneously inducing the second electrical current in thesame electrical direction with respect to the exciting electromagneticcoil around the path of the second conductivity cell as the firstelectrical current is flowing around the path of the first conductivitycell.
 3. A method as defined in claim 2 further comprising:alternatingthe electrical direction of each of the electrical currents in the firstand the second conductivity cells.
 4. A method as defined in claim 2wherein:the sensing and subtracting step further comprises: positioninga sensing electromagnetic coil in proximity with the first and secondconductivity cells at a sensing location, the first and secondconductivity cells being oriented at the sensing location with the firstelectrical current in an electrical direction with respect to thesensing coil opposite to the electrical direction of the secondelectrical current with respect to the sensing coil.
 5. A method asdefined in claim 4 wherein:the exciting electromagnetic coil defines awindow through which the first and the second conductivity cells pass;and the sensing electromagnetic coil defines a window through which thefirst and the second conductivity cells pass.
 6. A method as defined inclaim 1 wherein the sensing and subtracting steps furthercomprise:positioning a sensing electromagnetic coil in proximity withthe first and second conductivity cells at a sensing location, the firstand second conductivity cells being oriented at the sensing locationwith the first electrical current in an electrical direction withrespect to the sensing coil opposite the electrical direction of thesecond electrical current with respect to the sensing coil.
 7. Anapparatus for determining a difference in electrical conductivity of atleast two fluids comprising:a first conductivity cell adapted to containa first fluid and having a tubular ring configuration forming acontinuous fluid path; a second conductivity cell adapted to contain asecond fluid and having a tubular ring configuration forming acontinuous path; means for inducing a first electrical current in thefirst fluid in the first conductivity cell and a second electricalcurrent in the second fluid in the second conductivity cell; means forsensing the first electrical current in the first fluid in the firstconductivity cell and the second electrical current in the second fluidin the second conductivity cell and for subtracting the secondelectrical current from the first electrical current to,produce a signalrepresentative of the difference in the conductivity between the firstand the second fluids.
 8. An apparatus as defined in claim 7 wherein:theinducing means further comprises an exciting electromagnetic coil inproximity with the first and second conductivity cells at an excitinglocation; and the first and second conductivity cells are oriented withrespect to the exciting electromagnetic coil at the exciting location toinduce the first electrical current in an electrical direction withrespect to the exciting electromagnetic coil and to simultaneouslyinduce the second electrical current in the same electrical directionwith respect to the exciting electromagnetic coil.
 9. An apparatus asdefined in claim 8 wherein:the direction of each of the first and thesecond electrical currents alternates.
 10. An apparatus as defined inclaim 8 wherein:the sensing and subtracting means further comprises asensing electromagnetic coil in proximity with the first and secondconductivity cells at a sensing location; and the first conductivitycell is oriented at the sensing location the first electrical current inan opposite electrical direction with respect to the sensingelectromagnetic coil from the electrical direction of the secondelectrical current with respect to the sensing electromagnetic coil. 11.An apparatus as defined in claim 10 wherein:the exciting electromagneticcoil defines a window through which the first and the secondconductivity cells pass; and the sensing electromagnetic coil defines awindow through which the first and the second conductivity cells pass.12. An apparatus as defined in claim 7 wherein:the sensing andsubtracting means further comprises a sensing electromagnetic coil inproximity with the first and second conductivity cells at a sensinglocation; and the first conductivity cell is oriented with respect tothe second conductivity cell at the sensing location with the firstelectrical current in an opposite electrical direction with respect tothe sensing electromagnetic coil from the electrical direction of thesecond electrical current with respect to the sensing electromagneticcoil.